1. Field of the Invention
The present invention is related to a display device, and more particularly to a liquid crystal display using a cholesteric mixture.
2. Discussion of Related Art
Liquid crystal display (LCD) devices have become increasingly popular in recent years as prices have fallen, the devices have become thinner and more lightweight, and the picture qualities have improved. LCD devices are generally categorized as either reflective or transmissive. Reflective LCDs tend to require less power since they lack an internal light source, and rely on ambient light reflected from the images displayed on the screen. Transmissive LCDs have an internal source of light, a back light, to produce brighter images.
Transmissive LCD devices typically include one or more array substrates, liquid crystal layers, color filter substrates, and electrode layers. Voltages may be applied to the electrode layers to generate an electric field between them and align the molecules of the liquid crystal layer(s). In this way, light from the back light passing through the liquid crystal forms images as seen from the front of the transmissive LCD device.
Materials in a liquid crystal state tend to exhibit characteristics of matter which are between the solid phase state (crystalline) and the liquid state (isotropic). Materials in a liquid state exhibit isotropic properties, that is, properties that are the same regardless of the direction of measurement. Materials in a solid state are anisotropic in that they tend to have properties which vary depending on the direction of measurement. The liquid crystal phase state is a thermodynamically stable phase which exhibits anisotropic properties due to the alignment and the shape of the molecules in the liquid crystal material. The temperature and concentration of a material in the liquid crystal state affects the material's phase state, thus affecting its properties. There are a number of distinct types of liquid crystal states.
One liquid crystal phase state, the nematic liquid crystal phase, has been studied extensively and is found in common use in a number of LCD applications such as the flat screen monitors for laptop computers. The molecules in a nematic liquid crystal phase do not have positional order relative to each other, but tend to be directionally oriented in one common direction, along the director. One class of the nematic liquid crystal phase state is chiral nematic, sometimes referred to as cholesteric. Materials in a cholesteric state have can selectively reflect one component of circularly polarized light.
Another liquid crystal phase state is the smectic phase. Molecules in the smectic phase state tend to be oriented in a common direction along a director, like nematic state molecules. However, unlike nematic molecules, smectic molecules also tend to be aligned in planes or layers. Hence, smectic molecules are more solid-like than nematic molecules. Different combinations of smectic layers and/or cholesteric layers may be combined with electrodes and other components in various manners to produce flat screen LCD devices with a variety of capabilities.
U.S. Pat. No. 6,549,261 ('261 patent), issued to Okada et al. features a reflective LCD having a cholesteric liquid crystal capable of selectively reflecting spectral rays of a specific visible wavelength. In the '261 patent, either a carrier layer or the cholesteric liquid crystal itself contains a coloring agent which absorbs wavelengths different from the reflected wavelength of the cholesteric liquid crystal. The device displays visible rays of a specific wavelength which are observed as a specific color. The devices may be stacked to produce red, green and blue in various combinations, as shown in FIG. 2 of the '261 patent, and herein in FIG. 1A.
U.S. Pat. No. 6,693,689 ('689 patent), issued to Kim et al. features a reflective LCD which includes a linear polarizer to convert natural light into linearly polarized light, and a retardation film to convert the linearly polarized light into circularly polarized light. The devices described in the '689 patent has a liquid crystal layer used to vary the phase of the light according to an applied electric field, and a cholesteric liquid crystal color filter to selectively reflect light received from the liquid crystal layer, along with a black background for absorbing light passing through the color filter. The device has pixels (or subpixels) dedicated to red, green and blue as shown in FIG. 12 of the '689 patent, and herein in FIG. 1B.
U.S. Pat. No. 6,580,482 ('482 patent) issued to Hiji et al. features a reflective multicolor LCD device that includes a cell 51 having a display layer 31 with a right-handed cholesteric liquid crystal for blue, a cell 53 having a display layer 33 with a left-handed cholesteric liquid crystal for green, a cell 57 having a display layer 37 with a right-handed cholesteric liquid crystal for yellow, and a cell 55 having a display layer 35 with a left-handed cholesteric liquid crystal for red. The various cell layers laminated in a stack to produce the multiple colors as shown in FIG. 1 of the '482 patent and herein in FIG. 1C.